Urea products are products comprising urea and, preferably, also another component generally suitable for fertilizing purposes. Besides the known primary macro-nutrients phosphor and potassium, such components are, e.g., ammonium sulfate, ammonium nitrate, potassium chloride, or other salts based on sulfur, calcium salts, magnesium salts, zinc-salts (e.g. zinc oxide, zinc sulphate) and further salts of B, Cu, Fe, Cl, Mn, Mo.
This is reflected in the widespread existence of fertilizers that contain several of these nutrients, e.g. nitrogen and sulfur, in the same product. One such product is urea ammonium sulfate (UAS). An important advantage of UAS is that it can be produced in solid form. This facilitates the transportation of this fertilizer product over long distances, as well as its distribution over the land.
Urea granules are typically produced in a fluid bed granulator, wherein particles are kept in motion by an air stream. Onto these particles liquid urea is sprayed, in the form of droplets or as a film, thereby causing particle growth. The resulting granulate has excellent strength allowing it to be transported by ship or over land over large distances.
Ammonium sulfate is a salt which is generally produced by reacting ammonia with sulfuric acid. The resulting solution is concentrated and converted into solid particles or a slurry. Ammonium sulfate (AS) is soluble in urea (such as a urea melt) up to a concentration of about 20% and then forms a solution, i.e., a homogeneous liquid. This homogeneous liquid can be processed in a fluid bed granulator in essentially the same way as a standard urea solution.
A background reference relating to such a granulation process is US2013/0319060. Herein a method is disclosed for recycling ammonium salts, obtained from a scrubbing system for the removal of ammonia from the off-gas of a urea granulator, by mixing these salts homogeneously in the urea granulator. Both UAS (a urea/ammonium salt stream) and a urea solution are added to a granulator. Thereby the amount of UAS is the highest in the first compartment of the granulator, and decreased in downstream direction along the axis of the granulator. The highest amount of the urea solution is sprayed into the granulator at the granule flow outlet side, and the amount of urea solution is decreased in upstream direction along the axis of the granulator. The amount of ammonia salt is lower than 20% by weight. The method is disclosed to have the advantage that the moisture uptake of the resulting urea ammonium salt granules is similar to the moisture uptake of urea granulate.
It will be understood that the total content of ammonium sulfate in the granules produced according to the disclosed method, will be well below 20% by weight. In fact, the resulting product is disclosed to satisfy the specifications for urea. In practice, however, it is also desired to produce UAS having a substantially higher content of ammonium sulfate, particularly of over 20% by weight. This implies that UAS is to be produced wherein the concentration of ammonia sulfate would exceed the maximum soluble concentration. The production of UAS granules having such high concentrations of AS, typically greater than 20% by weight, is technically challenging due to, inter alia, a significant generation of dust.
Such a higher fraction of dust is released from the granules during many of the processing steps occurring in the granulation process (e.g. fluidization in fluidized bed granulator, fluidization in the granulate cooler, conveying, screening, etc.). All dust released in these processing steps is either entrained with the fluidization air (in granulator and/or in granulate cooler and/or in final product cooler) or entrained via a central dust removal system (vacuum cleaning) and washed out via a dust-washer. The wash water in which the UAS-dust particles readily dissolve consists of a solution of urea and ammonium sulfate in water and needs to be evaporated for further processing. All this is leading to an increase of the amount of UAS-solution that needs to be reworked, leading to higher capital and operational costs.
When UAS granulate with high AS-content is produced, the dust for the major part consists of Ammonium Sulfate, which puts extra constraints and demands to the rework/recycle of it. In general dust is scrubbed leading to a diluted UAS aqueous stream. The stream is reworked by concentrating in one or more evaporators. As it is desirable to process a homogeneous melt stream rather than a slurry stream, extra measures have to be taken to prevent that the solubility of ammonium sulfate in urea will be exceeded in these cases.
The increased dust-release/increased dust-content of the product will also lead to an increased tendency for fouling/scaling product build up at the inner walls of all process-equipment in the granulation process. This fouling will lead to an increased risk of choking/blocking and hence an increased necessity of cleaning, etc. This will lead to an increase of down time of the granulation plant.
During handling and processing steps downstream the granulation plant, the presence of dust will lead to various commonly known consequences such as caking, dusting, quality issues, piling, loss of product etc.
It is thus desired to provide a process enabling the production of UAS granules that is capable of producing UAS comprising an insoluble amount of ammonium sulfate, whereby one or more of the aforementioned problems are alleviated or eliminated.
Similar consideration apply to producing granules of other urea products, comprising one or more additives, such as a salt, in a proportion exceeding the solubility limit thereof.